Perimeter intrusion detection system with common mode rejection

ABSTRACT

A perimeter surveillance system is described for sensing intrusions into a specified area. The system includes a cable buried beneath the surface of the earth and along the perimeter of the area to be protected. Associated with the cable are a plurality of transducers electrically connected thereto and also buried beneath the surface of the earth. These transducers may be, for example, piezoelectric elements sensitive to soil stress in substantially one direction only and may be mounted in an arrowhead-shaped housing for ease of installation. Each piezoelectric transducer is connected to the buried cable with its polarity opposite to that of its nearest neighbors in order to cancel out undesired noise from remote sources. The transducers themselves are capable of broad-band response, from just above d.c. to several kHz. In one embodiment, the response of the total system is limited to sub-seismic frequencies, so that the system is not sensitive to seismic-band signals caused by wind, rain, hail, distant vehicular traffic, vibrating machinery and other remote natural and man-made disturbances.

Bound et al.

[ PERIMETER INTRUSION DETECTION SYSTEM WITH COMMON MODE REJECTION [75]lnventors: Lloyd R. Bound, Carrollton; Alfred C. Hunting; Peter N.Linden, both of Dallas, all of Tex.

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Mar. 23, 1972 [21] Appl. No.: 237,307

[52] US. Cl. 340/261, 340/272 [51] Int. Cl. G081) 13/10 [58] Field ofSearch 340/416, 261, 272;

[56] References Cited UNITED STATES PATENTS 3,109,165 10/1963 Bagno .l340/261 3,479,536 11/1969 Norris 310/8.1 3,688,251 8/1972 Morris 310/832,548,990 4/1951 MCLoad 310/82 2,836,737 5/1958 Crownover 3l0/8.13,613,061 10/1971 Lund 340/261 [111 3,806,907 [451 Apr. 23, 1974 PrimaryExaminer-John W. Caldwell Assistant Examiner-Marshall M. Curtis 57ABSTRACT A perimeter surveillance system is described for sensingintrusions into a specified area. The system includes a cable buriedbeneath the surface of the earth and along the perimeter of the area tobe protected. Associated with the cable are a plurality of transducerselectrically connected thereto and also buried beneath the surface ofthe earth.v These transducers may be, for example, piezoelectricelements sensitive to soil stress in substantially one direction onlyand may be mounted in an arrowhead-shaped housing for ease ofinstallation. Each piezoelectric transducer is connected to the buriedcable with its polarity opposite to that of its nearest neighbors inorder to cancel out undesired noise from' remote sources. Thetransducers themselves are capable of broad-band response, from justabove do. to several kHz. In one embodiment, the response of the totalsystem is limited to sub-seismic frequencies, so that the system is notsensitive to seismic-band signals caused by wind, rain, hail, distantvehicular traffic, vibrating machinery and other remote natural andman-made disturbances.

7 Claims, 17 Drawing Figures PATENTEDAPR23 um I 380630? sum 1 [1F 9MAXIMUM EFFECTIVE STRESS AS TARGET CROSSES LINE (HEAR) PATEMTEDAPR 23 mm3'. 8 063L907 sum 1 0r 9 PATH OF TARGET SENSOR CABLE EOUSSINESQFORMULAEZ W 3V 7 HORIZONTAL TRANSVERSE STRESS (TY 7 w 3x 2 LONGITUD'NALSTRESS PLUS POISSON s RATIO TERMS x 3 1| R5 0F SMALLER MAGNITUDE Y W \2VERTICAL STRESS Cl' 2000 EMPIRICALLY DETERMINED TO a'E,

AT MOST, ONLY A SHALLOW DIMPLE E 3 I H906 3w 2 Y z POISSON s ZUR s RATIOTERMSI TARGET WEIGHT: ISO POUNDS DEPTH OF SENSOR BURIAL: 0. 6 METERHORIZONTAL OFFSET FROM TRANSDUCER (METERS) THEMED APR 23 I974 SHEET 5 BF9 MTEMTEDAPRNIEJM 3806307 SHEEI7UF9 92 WIND NOISE ONLY. WITH COMMON MODEREJECTION TRAVERSES OF WALKER FAST WIND NOISE AND A STEALTHY SUCCESSIVETRANSDUCERS WALK PARALLEL TO AND OFFSET FROM THE LINE NO COMMON MODEREJECTION WIND NOISE TIME. I MM/SEC /'g 8 V WIND SPEED: IO TO I5 MPHF'ERPENDICULAR TO THE LINE Fig, /0/I TIME (SECONDS) A. SIGNATURE OFSTEALTHY TRAVERSE AT I METER OFFSET 1 TARGETS I80 POUNDS NOTE: FENCEEMBEDDED IN PAVING, WHICH EXTENDS PAST TRANSDUCER LOCATION SHAKING THEF- FENCE '1 TIME (SECONDS) B. RESPONSE OF SINGLE TRANSDUCER TOSIMULATION OF VERY STRONG WIND SHAKING CHAIN-LINK FENCE 25 FEET FROMTRANSDUCER PATENTEDAPRZEHHH SHEET 3 OF 9 mowmmooml N! 3 mmmonomzst.

239$ .55 .um mmmEImsE mmzho 20mm Tillllfillf v v m PATENTED 23 1974 sum9 [IF 9 PERlMETER-INTRUSION DETECTION SYSTEM WITH COMMON MODE REJECTIONThis invention relates to intrusion detection and more particularly to aperimeter intrusion detection system for detecting the occurrence ofintrusions across the perimeter of a protected area.

Of all the various modes used in the past for sensing the occurrence ofan intrusion in an outdoor environment, only a very few have been ableto provide a high degree of localization for coverage of a long narrowcorridor. Prior art systems utilized, for example, a magnetic sensor, acapacitance line sensor or a balanced pressure sensor. However, themagnetic sensor is appropriate only if the intruder is likely to becarrying ferromagnetic material, and the capacitance line is adverselyaffected by changes in the conductivity of its environment, such as soilmoisture changes. The balanced pressure sensor, comprised of twoparallel lines each filled with a liquid and connected to a device whichmeasures the pressure differential between the two lines, is verysusceptible to externally induced undesired noise, such as wind.

Accordingly it is an object of the present invention to provide aperimeter intrusion detection system for detecting intruders entering ordeparting from specified areas.

Another object is to provide a perimeter intrusion detection systemwhich is sensitive to quasi-static soil stress arising from near-field,slowly varying loading on the soil surface.

Another object of the invention is to provide a perimeter intrusiondetection system which is very insensitive to undesired noise induced byremote sources.

Another object of this invention is to provide a perimeter intrusiondetection system which is insensitive to wind, rain, hail, earthquakes,distant vehicular traffic, vibrating machinery and other remote naturaland man-made disturbances.

Another object of this invention is to provide a perimeter intrusiondetection system which is completely hidden from the intruder.

Another object of the present invention is to provide a transducer foruse with said perimeter intrusion detection system which is sensitiveonly to soil stress variations and mainly to variations in oneparticular direction.

Another object is to provide a transducer which is sturdily constructedand which lends itself to a novel and inexpensive method ofinstallation.

A further object is to provide a perimeter intrusion detection systemwhich is easy to install and maintain.

A still further object is to provide a highly-reliable.

perimeter intrusion detection system which includes no moving parts anda minimum of buried circuitry.

Other objects and features of the invention will be come more readilyunderstood from the following detailed description and appended claimswhen read in conjunction with the accompanying drawings, in which likereference numerals designate like parts throughout the figures thereof,and in which:

FIG. 1 shows a typical perimeter intrusion detection,

FIG. 2 is a schematic of the installed intrusion detection systemaccording to one embodiment of the present invention. I

FIG. 3 is an enlarged perspective break-away view of the sensor assemblywhich includes an arrowhead hous ing with the piezoelectric transducerassociated therewith.

FIGS. 4A and 4B illustrate a more detailed front and cross-sectionalside view of the piezoelectric transducer utilized in FIG. 3.

FIG. 5 shows the geometry of thesoil stress calculations utilized inconjunction with the transducer illustrated in FIGS. 4A and 48.

FIG. 6 is a graph which illustrates the calculated peak soil stress at asingle transducer for intruder traverses with various offsets from theburied transducer.

FIGS. 7A-7D illustrate the various steps in implanting the sensorassembly shown in FIG. 3.

FIG. 8 illustrates typical wind noise effects on the waveforms generatedin the perimeter intrusion detection system both with and without commonmode rejection.

FIG. 9 illustrates another embodiment of the perimeter intrusiondetection system according to the present invention.

FIGS. 10A and B illustrate the waveforms obtained with a sensoraccording to the present invention when buried below a paved surface,and when a strong wind shakes a nearby fence.

- FIG. 11 illustrates the schematic for the electronics used with eachsegment of the perimeter intrusion detection system.

FIG. 12 illustrates a typical alarm display and system control consolefor the perimeter detection detection system.

FIG. 1 shows a typical perimeter surveillance system 10 according to thepresent invention. The system 10 is provided to report intrusions nearthe fence 12 which surrounds the site 14 to be protected. The system 10can be designed such that an indicator or alarm is sounded each time aperson comes within a predetermined distance to the system, for example25 feet. It will be noted that while the illustration shows the areasurveillance system surrounding the outside of fence 12 to detectintrusions into the site l4, it would also be possible to include asimilar system 10 within the confines of fence 12 in order to detectexits from the site 14 as well. The system 10 consists of a buriedmulticonductor cable 16 composed of a plurality of segments 18 which areinterconnected by way of connectors 20. Electrically associated withcable 16 are a plurality of buried soil stress sensors 22. The cable 16is buried about six inches from the surface of the earth and the trenchenclosing cable 16 is back-filled and levelled for minimum visibility.Any intrusion across system 10 will be detected by the minute variationsin the stress of the surrounding soil to which sensors 22 are sensitive.These stress variations can be as slow'as a man stealthily crawling ortiptoeing over sensors 22 or they can be faster than a man running. Inresponse to anysuch intrusion, an alarm or other type of indication willbe transmitted to alarm display and system control console 24 inmonitoring station 26. Station 26 may be remote from the protected area.

FIG. 2 .is a more detailed schematic of a portion of the perimeterintrusion detection system 10 illustrated in FIG. 1. One segment 18 ofcable 16 isillustrated in FIG. 2. Each segment 18 consists of at leastone soil stress sensor 22; normally there are a large number of suchsensors located within one segment. Typically, segment 18 is buriedabout 6 inches below the surface of the earth, while sensors 22 arelocated between 6 and 36 inches below the surface of the earth. Atypical segment 18 can be as long as several hundred yards or as shortas desired, depending upon the resolution within which the intrusion byan intruder, such as human intruder 28, needs to be localized. Thisenables the system to indicate an intrusion on monitor 24 (FIG. 1) towithin any required resolution. Within a resolution element or segment18, each of the sensors 22 is connected in parallel to a pair of inputlines 30. The spacing between sensors is typically 1 meter. The sensors22 have flat sides terminating in a wedge shaped point and are orientedin a vertical plane 32 perpendicular to cable 16. Sensors 22 are alsooriented such that their stress-sensitive faces are all aligned in thesame direction indicated by the arrows S. With the sensors 22 depolyedin this manner, a lineal configuration is obtained with the sensors 22sensitive to stress variations along a relatively narrow corridor 33whose center isover the length of the cable 16. Accordingly,disturbances outside this narrow corridor do not cause alarms, whereasdisturbances within this corridor, such as a human intruder 28, willcause an alarm. A typical sensor'22 is connected through leader cable 34to input lines 30 with a polarity opposite to that of its nearestneighbors. This provides common-mode rejection, which means that thesystem 10 is sensitive to localized sources of soil stress butinsensitive to sources of stress that are nearly uniform over largeareas, such as wind, rain and other natural and man-made'phenomena. Thisfeature will be discussed in more detail later. The output signals fromeach of the sensors 22 are combined into a composite signal, which isfed into.an amplifier 35. This amplifier 35 and associated testcircuitry, one for each segment 18, is the only kind of circuitrycontained within the buried line. Thus the amount and complexity of thecircuitry that is not readily accessible is kept to a minimum.

FIG. 3 is a more detailed break-away view of the sensor 22 illustratedin FIG. 2. The sensor 22 has a housing 36, which may have the generalform of an arrowhead. The housing should be made of a tough durablematerial and may be molded of apolycarbonate plastic, such as Lexan,which easily withstands the impacts and abrasions of installation, evenin a rocky soil. One end of the arrowhead has bevelled edges 38 whichform a point; the other end of the arrowhead has a drive socket 40molded therein. An opening 42 is located approximately at the midpointof the arrowhead for placement of transducer 44. Opening 42 may be ahole completely through the housing 36, in which case a seating shoulderinside the hole'is used to support the back of transducer 44. In FIG. 3the opening 42 is shown extending only part way into housing 36;accordingly, the back of transducer 44 is seated against the backsurface of opening 42. In either case, transducer 44 is sensitive tostress mainly from a single direction. A force-dividing aperture 46 islocated between the drive socket 40 and the opening 42. This protectsthe transducer 44 from undue stresses during installation. When sensor42 is implanted into the surface of the earth, the force impressed upon.the housing will be divided around aperture 46, and accordingly alsoaround transducer 44, as

shown by force arrows 48. The hole 50 extends from the side of thehousing 36 to the opening 42 in order to accommodate leader cable 34from cable 16 (FIG. 2). A pair of conductors 52 in leader cable 34connect transducer 44 to input lines 30 in cable 16 (FIG. 2). With thetransducer 44 in place in opening 42 and with the sensitive surface ofthe transducer facing outward, a suitable material, such aspolyurethane, is utilized to fill opening 42. This material serves bothto hold the transducer 44 in place and to transfer the soil stress totransducer element 44.

FIGS. 4A and 4B 'are 'a front view and a crosssectional side view of thetransducer 44 utilized in the housing 36 (FIG. 3).-Transducer 44, inthis embodiment, is a piezoelectric type transducer. A variablereluctance transduceroran electrodynamic transducer are examples ofother types of transducers that could also be used and which aresensitive to changes in the soil stress.

In FIGS. 4A and 4B, transducer 44 has a transducer support structure 54for supporting bender plate 56. The support structure 54 is made ofplastic, while bender plate 56 is made of brass. A piezoelectric crystaldisk58, typically made of an alloy of lead zirconate and lead titanate,has tin-lead coatings formed on both its sides. One side of disk 58 isin turn soldered to bender plate 56. One of the leads S2 in leader cable34 (FIG. 3) is connected to the positive side formed on the tin-leadcoating 60 on the top of the piezoelectric disk 58, while the otherconductor 52 is connected to the negative contact 64 formed on benderplate 56. Bender plate 56 provides structural support for thepiezoelectric disk 58 and further supplies additional restoring forcewhen the piezoelectric crystal disk 58' is deformed by the soilstresscreated by an intrusion. The cavity 66 (FIG. 4B) behind benderplate 56 allows the plate and the piezoelectric'disk 58 to deform freelyin response to stress (shown by arrow 68) applied normal to thefront'face of transducer 44. The high compliance this gives to the diskand the bender plate provides the equivalent of considerable mechanicaladvantage in coupling the external stress supplied by the weight of anintruder to the piezoelectric disk 58, thus making possible a relativelyhigh value of transducer sensitivity. This form of piezoelectrictransducer responds mainly to stress perpendicular to'the sensitivesurface (such as is illustrated by stress arrow 68).

The soil stress sensor (illustrated in detail in FIGS. 4A and 4B)responds almost exclusively to one of the few characteristics theintruder is certain to exhibit, i.e., weight which is supported by theground surface he is traversing. Accordingly, a sensor, such astransducer 44, is desirable for detecting the occurrence of intrusionsacross a perimeter around a site to be protected. The frequencies oftheisoil stress variations that are of interest are quasi-staticfrequencies, i.e. frequencies which extend from just above dc up to 5 to10 Hz, just below the range where seismic signals from distant sourcesbecome significant. Transducer 44 measures soil stress variations andmainly the soil stress in the direction of the sensor cable 16. All ofthe transducers 44 (shown more clearly in outline in FIG. 2) have theirsensitive surfaces facing in the same direction, which is along theline'of the sensor cable 16, so that a narrow corridor 33 (also FIG. 2)is protected. Referring now to FIG. 5, transducer 44 provides a highdegree of localization transverse to' the line of the sensor cable 16,

by virtue of the sharp falloff of soil stress, and thus of transducerresponse, as the point of application of the intruders weight W onground surface 70 moves away from the ground-zero point of thetransducer 44. The geometry of this configuration is shown in this FIG.5.

' Here the origin of the coordinate system is the point of applicationof the surface loading. The Boussinesq formulas for the soil stressesinduced by a point load on the surface 70 illustrated in FIG. 5 werederived on the basis that'soil is an isotropic elastic medium and thatthe weight of the soil itself can be disregarded in comparison withexternally applied forces, such as the weight W of an intruder. Itshould be noted that the stress 0-,, (denoted in the formulae) in thedirection of the sensor cable 16 theoretically should go to zero whenthe weight load is located directly above transducer 44. However,inclusion of'soil compressibility ef fects, by way of the Poissons-ratioterms, transforms the zero region from the x-axis to a nearly ellipticalcurve centered around the x-axis. The results of field testing show theabsence of a zero or even a deep minimum in the vicinity of the groundzero point (i.e., directly above transducer 44). This is attributed toinhomogeneities in the intervening soil medium and to the spreading ofthe stress distribution patterns because of the finite surface area 70on which the weight W is applied.

FIG. 6 shows calculated values of'the peak stress, in microbars,-attransducer 44 when the traversing target, such as intruder 28 (FIG. 2),is directly above the cable 16 and at various offsets from thetransducer 44. FIG. 6 shows that as the offset of the intrusion pathfrom the sensor 22 increases, the stress exerted perpendicular to sensor22 drops off. Since the narrow corridor 33 (FIG. 2)ext ends on bothsides of system 10, a typical corridor of sensitivity would be about 4to 6 meters wide, i.e., 2 to 3 meters on either side of cable 16.

FIGS. 7A-7D illustrate a novel method of installation of the sensorshown in FIG. 3. A narrow trench 72, roughly 6 inches by 6 inches, isdug with a trencher (not shown) along the line where the perimeterintrusion detection system 10 is to be placed (see FIG. 7C). The mainconnecting cable 16, with arrowhead shaped sensors 22 attached to leadercables 34, is laid out in trench 72. In FIG. 7A, implanting shaft 74 hasone end 76 which mates with drive socket 40 located in the top of sensor22. The other end 78 of implanting rod 74 mates with a driving mechanism80, such as an air hammer. FIG. 7B shows sensor 22, implanting rod74.and air hammer 80 interconnected in their proper relationship. InFIG. 7C, the sensor 22 is shown ready to be implanted with air hammer 80into the soil at the bottom of trench 72. A depth indicator, such asstripe 82 on implanting rod 74, denotesthe proper depth to which sensor22 is to be driven. FIG. 7D shows sensor 22 driven into the soil to thedepth indicated by stripe 82 on implanting rod 74. The depth ofimplantation is typically inches and a typical spacing between sensors22 is about 40 inches. Each of the arrowhead shaped sensors 22 should beoriented properly with the flat sides of said sensor oriented in avertical plane (such as plane 32 in FIG. 2) perpendicular to cable 16.If in some cases the soil is too rocky for the arrowhead sensor 22 tofind any paths straight down, then a jack hammer may be utilized to opena vertical hole through the tional 3 to 5 inches in the mannerillustrated in FIGS. 7A-7D, so that the sensitive surface of thetransducer is firmly embedded in undisturbed soil. After the arrowheadsensors have all been implanted, shallow trench 72 is refilled and thesoil surface leveled and smoothed for minimum visibility.

A particular advantage of the method of installation illustrated inFIGS. 7A-7D is that when the arrowhead sensor 22 is driven straight downinto previously undisturbed soil and both its side surfaces becometightly embedded into the soil, there is little'possibility for theformation of a loose interface with the soil around it. Loose interfacescan occur with other modes of implantation in the soil; in such cases,the sensitive surface of the transducer can become decoupled from thesoil medium and thereby lose sensitivity, especially to low-amplitudesignals.

Because transducer 44 can sense only low-frequency soil stresses, falsealarms can arise only from sources that can exert stress directly on thetransducer face by loading the soil nearby, either at the soil surfaceor at a subsurface interface. The only such source of importance thathas pressure levels and frequencies of variation that resembleintrusions is strong gusts of wind. These are coupled to thesoil aroundthe transducer both directly, by a fluctuating pressure on the soilsurface, and indirectly, by wind-induced vibrations in nearby trees andabove-ground structures, which exert forces and torques on soilinterfaces near the transducer. According to the present invention, aneffective way of dealing with wind-induced noise is to make use of themain difference between actual intrusions and the effects of the wind,i.e., localization. The direct loading of the ground surface by windgusts is spread over an area much larger'than the spacing between thetransducers. Similarly, if a structure whose foundation is coupling thewind gusts into the soil is much farther from the system 10 than thespacing between transducers 44, then the strength and direction of thesestresses at any given instant will be nearly the same at neighboringtransducers. Accordingly, as shown in FIG. 2 and described with FIG. 8,transducers 44 utilize commonmode rejection, in which each transducer 44is connected to its common input line 30 (FIG. 2) with a polarityopposite to that of its nearest neighbors. Thus, if an externalperturbation has essentially the same amplitude and phase at severalneighboring transducers 44, it will produce essentially no net signal atamplifier 35 (FIG. 2).

' Referring now to FIG. 8, the effect of common-mode rejection inreducing the effect of wind noise and other perturbations can be seen bycomparing the signals obtained in an actual test environment. Twoadjoining segments of cable were used. In one length,transducers 44 wereall connected with the same polarity; in the, other, transducers 44 wereconnected with alternating polarities for common-mode rejection. In FIG.8, waveform 86 is thesignal obtained with no common-mode rejection andthe wind noise at point 88 can be seen; peaks 90 represent intrudertraverses past successive transducers 44. Waveform 92 is the outputsignal from transducers 44 with common-mode rejection. Wind noise is theonly perturbing source here; there are no intruder signals present.Waveforms 88 and 92 show that the wind noise is reduced by almost anorder of magnitude by the use of common-mode rejection. These data weretaken with lines in which the transducer spacing was 4 meters. With 1meter spacing, a signal-to-noise ratio in the order of 30:1 or higher isachievable, even in the presence of to 30 mph winds.

It might be expected that the reversal of polarity for each successivetransducer would give rise to a line of zero net sensitivity on theground surface half way between each successive pair of transducers.Field tests have been conducted in which traverses were made at closelyspaced offsets progressing from one transducer toward its neighbor. Inthese tests, the main interest was in the general shape of the rise andfall of the signal level as the walker or intruder crossed the line, andin the variation of the maximum amplitude of the signal as the traversesapproached the point midway between transducers. These tests showed thatthe maximum amplitudes generally followed the calculated stress curveshown in FIG. 6 with a relatively smooth transition at the midpoint. Theabsence of a zero or a deep minimum is attributed to inhomogeneities inthe soil and to irregularities in the footprint loading pattern.

FIG. 9 illustrates another embodiment of the perimeter intrusiondetection system 10 illustrated in FIG. 1. For simplicity, the referencenumbers used in FIG. 9 are the same as for the corresponding items inthe first embodiment of FIG. 1. In FIG. 9, the perimeter surveillancesystem 10 consists of a lineal array of transducers, such as, forexample, the piezoelectric transducers 44 illustrated in FIGS. 4A and4B. Around these, a common jacket 84 is extruded forming a singleflexible cable 16. This is buried along the perimeter of the site to beprotected. Basic resolution elements or segments 18 are connected inseries, through connectors 20, with other identical segments to form acontinuous perime ter intrusion detection system such as isdescribedwith reference to FIG. 2. The transducers 44 are typically evenly spacedwithin the cable jacket 84. Signal conditioning circuitry (not shown,but similar to amplifier 35 in FIG. 2) is located at the end of eachsegment 18. The jacket 84 may be made of polyurethane or other similarmaterial. The polyurethane jacket 84'should be in intimate contact witheach of the transducers 44, in order to insure that the transducers 44are in intimate mechanical contact with the soil around the cables.Otherwise, the sensitive surface of the transducer can become decoupledfrom the soil medium and thereby lose sensitivity to low amplitudesignals. As in the system shown in FIG. 2, the embodiment shown in FIG.9 has the sensitive surface of the transducer 44 perpendicular to theline of cable 16. Similarly, common-mode rejection is provided by havingeach of the transducers 44 connected to a common input line (not shown)with a polarity opposite to that of its nearest neighbors. This approachallows the system to reject noise produced by large-area and by remotesources, such as wind, rain, distant vehicular traffic and vibratingmachinery.

The concepts utilized in the perimeter intrusion detection systemaccording to the present invention are equally applicable under an areahaving a paved surface. The theory of soil stresses indicates that thestress due to a'weight load on a paved surface should be different fromthe unpaved case only to the extent that the paved surface is stifferthan the soil beneath it and tends to spread out the distribution of theload on the surface. Accordingly, the signature or waveform of atraverse on a paved surface is nearly the same as on a soil surface.FIG. 10A shows the signal produced by a stealthy traverse of an intruderat an offset of 1 meter. The transducer was embedded about8 inches intothe soil beneath a 4-inch thick surface layer of asphalt paving. Thesignature shown in FIG. 10A confirms the similarity to the unpaved case.

It is also of concern whether a paved surface will transmit soilstresses from distant wind-shaken structures, such as fence 12 (FIG. 1and 2), differently than does uncovered soil. A simulation was made ofthe effect of the wind bymanually shaking a chain-link fence located 25feet from the transducer. This fence was anchored through the pavedsurface which extends to and beyond the transducer. The relatively lowamplitude of the transducer output, as shown in FIG. 108, indicates thatwind-induced noise from this source is not a significant problem.

FIG. 1 1 shows the circuitry utilized with a typical segment of theperimeter surveillance system according to the present invention. Thecircuitry consists of input section 94, a two-stage amplifier 96, aprocessor 98 and a display and/or alarm system 100. In FIG. 2, all thetransducers 44 in a segment 18 of cable 16 are connected in paralleltooutput lines 30. FIG. 11 shows the successive transducers 44 in inputsection 94 connected with alternating polarity, to provide thecommon-mode rejection feature previously described.

Amplifier 96 has two stages, a charge-amplifier stage and a voltage-gainstage. The output signalfrom the charge amplifier stage has an amplitudewhich is proportional to the displacement of charge in the input circuitand is essentially independent of the value of the capacitance inparallel with the input. Here the displacement of charge is produced bytransducers 44 in response to variations in the soil stress caused bythe intruder 28. The charge amplifier stage features a low inputimpedance and an immunity to the effects of stray capacitance in theinput circuit. The principle of operation of the charge amplifier stageis similar to that of a conventional operational amplifier. The maindifference is that at the input'terminal pin 2 of operational amplifierU1, the input charge from the transducers 44 in input section 94 isbalanced by the charge on capacitor Cl that is fed back from the outputof the amplifier. Feedback resistor R2 provides-dc rolloff, so the.amplifier does not appear as an open-loop integrator. Resistor R1 isdetermined by the required high-frequency cut-off. The low-frequency andhigh-frequency 3-db points f and f are given by:

f l/R C ;f l/R C where C is the value of the transducer capacitance andC, is the value of the feedback capacitance. The midband gain k of thecharge amplifier stage is determined by The transfer function for thecharge amplifier stage is f is in the sub-seismic frequency range,typically less than 10 Hz. This keeps the perimeter surveillance systemrelatively insensitive to noise sources in the seismic frequency range.The system is sensitive only to stress variations in the sub-seismicrange, typically between 0.01 to 10.0 Hz. These stress variations aredue almost entirely to the quasi-static soil pressure arising from thenear-field, slowly-varying loading on the soil surface. The low-passfiltering provided by the charge amplifier stage could be provided by aseparate filter located after amplifier 96.

The second stage of amplifier 96 is a conventional voltage gain stage.The output of this stage is fed to processor 98, along with the outputsfrom similar amplifiers in other segments (not shown) of the cable.

Typical values for the electronic components of the two-stage amplifier96 are as follows:

Rl k fl. C4 22 p.F

R3 M!) C6 22 piF R5 IOMO. CRI IN9I4 R7 100!) CR3 6.2V Zener R8 1000 CR46.2V Zener Cl .047 F U1 UC4250C C2 33 pF U2 UC4250C C3 l6 pF Theprocessor 98 may be as simple as a threshold detector circuit or ascomplex as a digital computer, such as the Model 960A computermanufactured and sold by Texas Instruments Incorporated. The processor98 operates on the data supplied it by the various amplifiers in theperimeter surveillance system. It sends an output signal to the alarmdisplay and system control 100 when an intrusion has occurred.

System 100 may include one or more of the following features: (I) adevice for providing a permanent printed plain-language record of allalarms, control functions, and operator actions; (2) a control center bywhich any or all segments of the intrusion detection system can be putinto an alarm or an inhibit mode of operation; and (3) one or more mapdisplays to give an indication of the number and location of allintrusions across the monitored corridor. This display will also showwhich segments are currently in the inhibit mode. The map display may bea large area wall mount unit, and/or a smaller console unit, such asconsole 24 in FIG. 1; either or both types of displays can be remotelylocated from the monitored corridor.

Referring to FIG. 12, a typical display and control console 102 is shownin more detail. Console 102 could be utilized with the systemillustrated in FIG. 1. The lighted map display portion 104 of theconsole consists of a map overlay of the site to be protected (in thisinstance the outer perimeter of fence 12, see FIG. 1) andfurther'provides a visual status indication for each resolution elementor segment 18 (FIG. 2). An exploded view of insert 106 shows a possibleconfiguration of columns ofindicator lamps 108 and 110 which may denotean inhibit mode condition and an alarm condition, respectively, for eachresolution element along the site to be protected. The inner column ofindicator lamps 108 may be yellow lines of light which indicate aresolution element is in the inhibit mode along with a numeric readoutdevice 112 which indicates the number of the segment which has beenplaced in this mode. A flashing red line of light from one or moreindicator lamps 110 and a similar numerical readout from alarm numericreadout device 114 would indicate that an intrusion had taken placewithin corridor 33 in one or more segments 18 (FIG. 2).

Console 102 may also include control keyboards 116 and a teleprinter 118 to record all alarms, control functions and operator actions. Aspreviously mentioned, 6

that are in an alarm condition. Some of the control keyboardinstructions on keyboards 116 that can be utilized in controlling theperimeter surveillance system are listed below:

KEY OPERATION INHB Tells the processor to inhibit the element orelements indicated by the numbers entered, following the INHB command.

ACT Tells the processor to activate the resolution element(s) indicatedby the numbers entered following the ACT command. An automatic self-testmay be provided to test each resolution element after it is reactivated.

ST Tells the processor to self-test the element or elements indicated bythe numbers pressed,

following the ST command.

ALL Causes the preceding command to be performed on all segments of thesystem.

CLR Cancels an improper command.

EXC Tells the processor to execute a command. v

To Tells the processor to operate on all elements between two elementnumbers'inserted.

OFF Turns off the alarm of the specified resolution element(s). 1

It will be appreciated that although the perimeter intrusion detectionsystem 10 shown in FIG. 1 completely surrounds the site to be protected,the principles in-. volved in this system are applicable to one segmentof the cable with but one sensor therein and that the system 10 may takeany geometrical configuration, such as a line, rectangle, etc.

Although the present invention has been shown and illustrated inconnection with certain specific embodiments thereof, it is to beunderstood that further modifications may now suggest themselves tothose skilled in the art and it is intended that this invention shallinclude such modifications as come within the scope of the appendedclaims.

What is claimed is:

l. A perimeter intrusion detection system for sensing intrusions acrossa perimeter comprising:

a. at least one segment of cable to be buried beneath the surface of theearth including a pair of electrical conductors;

b. a plurality of soil stress sensors each comprising a transducerselectively responsive to pressure to be buried in a spaced relation oneto another for selective activation by a lateral pressure force, each ofsaid sensors having positive and negative terminals electricallyinterconnected with said pair of electrical conductors of the cablesegment with a polarity opposite to that of an adjacent sensor, wherebywhen a noise stress activates adjoining sensors to substantially equaldegrees the output of one sensor opposes the output of the adjacentsensor to alleviate false alarms, and when a sufficiently largelocalized pressure force activates essentially only one sensor anelectrical signal representative of the pressure force is produced; and

c. an electrical signal indicating means positioned remotely to saidsensors and operatively responsive to the electrical signalrepresentative of the pressure force to indicate the presence of anintrusion.

2. The system of claim 1 wherein all the soil stress sensors areimplanted in the earth at approximately the same depth and all face thesame direction along said cable segment.

6. The system of claim 1 wherein each soil stress sensor is oriented ina substantially vertical plane perpendicular to the at least one cablesegment whereby said soil stress sensor is operatively responsive tosoil stress variations in substantially only one direction.

7. The system of claim 1, wherein the spacing between said plurality ofsoil stress sensors is less than 8 feet.

1. A perimeter intrusion detection system for sensing intrusions acrossa perimeter comprising: a. at least one segment of cable to be buriedbeneath the surface of the earth including a pair of electricalconductors; b. a plurality of soil stress sensors each comprising atransducer selectively responsive to pressure to be buried in a spacedrelation one to another for selective activation by a lateral pressureforce, each oF said sensors having positive and negative terminalselectrically interconnected with said pair of electrical conductors ofthe cable segment with a polarity opposite to that of an adjacentsensor, whereby when a noise stress activates adjoining sensors tosubstantially equal degrees the output of one sensor opposes the outputof the adjacent sensor to alleviate false alarms, and when asufficiently large localized pressure force activates essentially onlyone sensor an electrical signal representative of the pressure force isproduced; and c. an electrical signal indicating means positionedremotely to said sensors and operatively responsive to the electricalsignal representative of the pressure force to indicate the presence ofan intrusion.
 2. The system of claim 1 wherein all the soil stresssensors are implanted in the earth at approximately the same depth andall face the same direction along said cable segment.
 3. The system ofclaim 1 wherein each of the plurality of soil stress sensors along saidcable segment is connected in parallel to the pair of electricalconductors of the cable segment.
 4. The system of claim 1 wherein theplurality of soil stress sensors are oriented in a vertical planeperpendicular to said cable segment.
 5. The system of claim 1 whereinsaid plurality of spaced soil stress sensors are located within saidcable segment.
 6. The system of claim 1 wherein each soil stress sensoris oriented in a substantially vertical plane perpendicular to the atleast one cable segment whereby said soil stress sensor is operativelyresponsive to soil stress variations in substantially only onedirection.
 7. The system of claim 1, wherein the spacing between saidplurality of soil stress sensors is less than 8 feet.